1. Field of the Invention
This invention relates to a method for the preparation of a cathode active material, capable of reversibly doping/undoping lithium, and to a method for the preparation of a non-aqueous electrolyte cell employing ths cathode active material.
2. Description of Related Art
Nowadays, in keeping up with the recent marked progress in the electronic equipment, researches into re-chargeable secondary cells, as power sources usable conveniently and economically for prolonged time, are underway. Representative of the secondary cells are lead accumulators, alkali accumulators and non-aqueous electrolyte secondary cells.
Of the above secondary cells, lithium ion secondary cells, as non-aqueous electrolyte secondary cells, have such merits as high output and high energy density. The lithium ion secondary cells are made up of a cathode and an anode, including active materials capable of reversibly doping/undoping lithium ions, and a non-aqueous electrolyte.
As the anode active material, metal lithium, lithium alloys, such as Lixe2x80x94Al alloys, electrically conductive high molecular materials, such as polyacetylene or polypyrrole, doped with lithium, inter-layer compounds, having lithium ions captured into crystal lattices, or carbon materials, are routinely used. As the electrolytic solutions, the solutions obtained on dissolving lithium salts in non-protonic organic solvents, are used.
As the cathode active materials, metal oxides or sulfides, or polymers, such as TiS2, MoS2, NbSe2 or V2O5, are used. The discharging reaction of the non-aqueous electrolyte secondary cells, employing these materials, proceeds as lithium ions are eluated into the electrolytic solution in the anode, whilst lithium ions are intercalated into the space between the layers of the cathode active material. In charging, a reaction which is the reverse of the above-described reaction proceeds, such that lithium is intercalated in the cathode. That is, the process of charging/discharging occurs repeatedly by the repetition of the reaction in which lithium ions from the anode make an entrance into and exit from the cathode active material.
As the cathode active materials for the lithium ion secondary cells, LiCoO2, LiNiO2 and LiMn2O4, for example, having a high energy density and a high voltage, are currently used. However, these cathode active materials containing metallic elements having low Clarke number in the composition thereof, are expensive, while suffering from supply difficulties. Moreover, these cathode active materials are relatively high in toxicity and detrimental to environment. For this reason, novel cathode active materials, usable in place of these materials, are searched.
On the other hand, it is proposed to use LiFePO4, having an olivinic structure, as a cathode active material for the lithium ion secondary cells. LiFePO4 has a high volumetric density of 3.6 g/cm3 and is able to develop a high potential of 3.4 V, with the theoretical capacity being as high as 170 mAh/g. In addition, LiFePO4 in an initial state has an electro-chemically undopable Li at a rate of one Li atom per each Fe atom, and hence is a promising material as a cathode active material for the lithium ion secondary cell. Moreover, since LiFePO4 includes iron, as an inexpensive material rich in supply as natural resources, it is lower in cost than LiCoO2, LiNiO2 or LiMn2O4, mentioned above, while being more amenable to environment because of lower toxicity.
However, LiFePO4 is low in electronic conduction rate, such that, if this material is used as a cathode active material, the internal resistance in the cell tends to be increased. The result is that the polarization potential on cell circuit closure is increased due to increased internal resistance of the cell to decrease the cell capacity. Moreover, since the true density of LiFePO4 is lower than that of the conventional cathode material, the charging ratio of the active material cannot be increased sufficiently if LiFePO4 is used as the cathode active material, such that the energy density of the cell cannot be increased sufficiently.
So, a proposal has been made to use a composite material of a carbon material and a compound of an olivinic structure having the general formula of LixFePO4 where 0 less than xxe2x89xa61, referred to below as LiFePO4 carbon composite material, as a cathode active material.
Meanwhile, as a method for the preparation of the LiFePO4 carbon composite material, having the olivinic structure, such a method has been proposed which consists in mixing lithium phosphate (Li3PO4) and iron phosphate I (Fe3(PO4)2 or hydrates thereof ((Fe3(PO4)2xc2x7nH2O), where n denotes the number of hydrates, adding carbon to the resulting mixture and in sintering the resulting mass at a pre-set temperature, such as 600xc2x0 C. or thereabouts.
However, Fe in LiFePO4 is in the bivalent state and is liable to oxidation, so that, at a synthesizing temperature of 600xc2x0 C. or thereabout, the following reaction of the chemical formula (1):
6LiFePO4+3/2O2xe2x86x922Li3Fe2(PO4)3+Fe2O3xe2x80x83xe2x80x83(1) 
occurs in air. As may be seen from this chemical formula, if the above compound is fired in an ordinary atmosphere, that is in air, impurities, such as trivalent Fe compounds, are produced, such that the LiFePO4 carbon composite material cannot be synthesized in a single phase.
It is therefore an object of the present invention to provide a method for the preparation of a cathode active material according to which the LiFePO4 carbon composite material can be synthesized in a single phase satisfactorily to realize satisfactory cell characteristics.
It is another object of the present invention to provide a method for the preparation of a non-aqueous electrolyte cell which, through use of the so-produced LiFePO4 carbon composite material, as the cathode active material, is superior in cell characteristics, such as cell capacity or cyclic characteristics.
In one aspect, the present invention provides a method for the preparation of a cathode active material including mixing, milling and sintering materials for synthesis of a compound represented by the general formula LixFePO4, where 0 less than xxe2x89xa61, and adding a carbon material to the resulting mass at an optional time point in the course of the mixing, milling and sintering, employing Li3PO4, Fe3(PO4)2 or hydrates Fe3(PO4)2xc2x7nH2O thereof, where n denotes the number of hydrates, as the materials for synthesis of the LixFePO4, and setting the oxygen concentration in a sintering atmosphere to 1012 ppm in volume or less at the time point of sintering.
Since the oxygen concentration in the sintering atmosphere is defined as described above, oxidation of Fe during sintering can be prevented from occurring and hence the single-phase synthesis of the LiFePO4 carbon composite material can be achieved satisfactorily even if the sintering is performed at a temperature e.g., of 600xc2x0 C. Meanwhile, the milling herein denotes executing the comminuting and mixing simultaneously.
In another aspect, the present invention provides a method for the preparation of a non-aqueous electrolyte cell including a cathode having a cathode active material, an anode having an anode active material and a non-aqueous electrolyte, wherein, in preparing the cathode active material, sintering starting materials for synthesis of a compound represented by the general formula LixFePO4, where 0 less than xxe2x89xa61, are mixed and milled, and wherein a carbon material is added to the resulting mass at an optional time point in the course of the mixing, milling and sintering. Li3PO4, Fe3(PO4)2 or hydrates Fe3(PO4)2xc2x7nH2O thereof, where n denotes the number of hydrates, is used as the starting materials for synthesis of the LixFePO4. The oxygen concentration in a sintering atmosphere is set to 1012 ppm in volume or less at the time point of sintering.
Since the oxygen concentration in the sintering atmosphere is defined as described above, oxidation of Fe during sintering can be prevented from occurring and hence the single-phase synthesis of the LiFePO4 carbon composite material can be achieved satisfactorily even if the sintering is performed at a temperature e.g., of 600xc2x0 C. So, with the use of this composite material as the cathode active material, a non-aqueous electrolyte cell which is superior in cell characteristics may be produced.